Optical disks such as DVD-ROMs and BD-ROMs have been widely used as media for recording digital content data. As to a DVD-ROM, a single-layer disk has a recording capacity of 4.7 GB (gigabyte), whereas a dual-layer disk with two recording layers has a recording capacity of 8.5 GB. As to a BD-ROM, a single-layer disk has a recording capacity of 25 GB and a dual-layer disk has a recording capacity of 50 GB. Therefore, the optical disks are becoming more and more popular as distribution media of digital data. Especially the BD-ROMs on which a large amount of data can be recorded are utilized as optical disks for readily distributing high-definition image content data.
As the recording capacity of an optical disk increases, the volume of an information content saved in a single optical disk and the value per optical disk have been heightened. This has brought about growing demand for distinguishing and managing, one by one, optical disks in which information contents are recorded by concave-convex pits.
Generally, however, one of the features of such optical disks that have information contents recorded thereon by concave-convex pits is that many copies of the optical disks can be produced at once from a master disk called stamper. However, due to such a feature of producing a number of copies from the master disk, the optical disks that are produced from this master disk have the completely same pit shape. It is, therefore, impossible to distinguish these optical disks from one another.
As a technology for identifying each of the optical disks produced by the stamper, there is disclosed a technology for recording an ID unique to a disk by providing a special area called “burst cutting area (BCA)” and recording a radially long mark in the form of a barcode by using a high-power laser called “YAG laser” (see Patent Literature 1, for example).
There is also disclosed a technology for performing pulse recording with very short channel clock unit width in the vicinity of a pit or land of a predetermined length or more on an optical disk and locally changing the reflectance ratio, to record disk identification information (see Patent Literature 2, for example).
Furthermore, there is disclosed a technology in which areas on an optical disk are divided into a data area and an identification area, wherein a plurality of pits and a plurality of lands are repeatedly recorded in the identification area to obtain predetermined modulated patterns, and the repeated pits and lands are irradiated with a gas laser beam to change a reflective film of the optical disk, from which a reproduction signal is detected (see Patent Literature 3, for example).
In addition, there is disclosed a technology in which the reflectance ratio of a reflective film is changed in order to record sub information, by radiating a laser beam continuously or intermittently onto concave-convex marks (concave-convex pits) with the integral multiple of the length of channel bits along the concave-convex marks, the concave-convex marks having recorded thereon randomly arrayed information contents that are main information (see Patent Literature 4, for example).
However, in the invention disclosed in Patent Literature 1, the paragraph 0035 describes that the reflective film of the optical disk is melted into a slit, which requires great energy in order to record the marks. Therefore, a relatively large and high-power industrial pulse laser such as YAG laser needs to be used for recording the marks. In other words, the recording device described in Patent Literature 1 is a large-scale recording device that is installed in a factory, and an optical disk drive for a general personal computer mounted only with a relatively low-power semiconductor laser light source cannot melt an aluminum reflective film to record an ID for identifying a disk.
In the invention disclosed in Patent Literature 2, the paragraph 0017 describes that, when recording the marks, a special device that turns out to be a finished device needs to be used in a factory to record the disk identification information in complete synchronization in units of channel bits of a disk reproduction signal. The pulse width for recording the disk identification information needs to be sufficiently shorter than a bit length in order not to degrade the quality of a reproduction signal of the information that are originally written in the pits. In the example described in Patent Literature 2, the recording pulse width corresponds to 1 channel bit. Normally when recording drastic pulses on highly precise positions on a reflective film having relatively good thermal conductivity, the pulse being detected stably by a reproducing device, a laser drive circuit that is as precise as or more precise than a recording optical disk device and a laser light source for outputting relatively large laser power are required.
In the invention disclosed in Patent Literature 3, the paragraph 0027 describes that an argon laser, He—Cd laser, or other type of gas laser is required as a laser source, which means that a special recording device managed in a factory is used, as with Patent Literature 1 and Patent Literature 2. According to the invention disclosed in Patent Literature 3, a pattern with a plurality of predetermined pits and lands needs to be prepared in a specific area of an optical disk. FIGS. 11A and 11B of Patent Literature 3 illustrate repeated patterns of pits and lands of a length corresponding to three channel bits (3T). Moreover, Patent Literature 3 does not specifically disclose what kind of physical properties are changed and how the reproducing device detects the reproduction signal when the reflective film is irradiated with a gas laser and consequently the properties of the reflective film are changed.
In the invention disclosed in Patent Literature 4, concave-convex pits on a certain area are irradiated with a laser beam to change the reflectance ratio of the optical disk somewhat extensively. The invention disclosed in Patent Literature 4 has the characteristics of easily obtaining detection sensitivity even when the amount of change in the reflectance ratio is low, because the reflectance ratio needs to be changed somewhat extensively. Patent Literature 4 describes that the marks are recorded by radiating a laser beam onto the concave-convex pits but does not describe a specific configuration of a recording device.